The present invention relates to rechargeable electrochemical energy storage systems, particularly such systems including electrochemical cells comprising complementary electrodes capable of reversibly intercalating, alloying, or otherwise alternately combining with and releasing lithium ions in electrical energy charge and discharge operations. The invention comprises, in its preferred embodiments, high capacity lithium battery cells comprising hydrated iron phosphate electrodes which provide stable discharge capacity in such cells.
Carbonaceous electrode materials, such as petroleum coke, hard carbon, and graphite, have been widely investigated and are regularly employed as active electrode materials in lithium and lithium-ion cells, but these materials are limited in volumetric capacity and present other difficulties, such as their contributing to the instability and degradation of electrolyte compositions.
More recently, three-dimensionally structured compounds comprising polyanions, e.g., (SO4)nxe2x88x92, (PO4)nxe2x88x92, (AsO4)nxe2x88x92, and the like, have been proposed to replace the simple oxides as viable alternatives to LiMxOy electrode materials. For example, the electrochemical functionality of such potential electrode materials as ordered crystalline olivine-form LiMPO4 compounds comprising transition metal cations, such as Mn, Fe, Co or the like, has been discussed at length by Goodenough et al. in U.S. Pat. No. 5,910,382. Similar functionality of other lithiated complex transition metal (PO4)nxe2x88x92 compounds has been discussed by Barker et al. in U.S. Pat. No. 5,871,866. Although these classes of crystalline lithiated phosphate compounds have exhibited some promise as electrode components, they possess significant drawbacks, such as an electrically insulating nature typified in the prevalent preferred materials, e.g., LiFePO4, and the fact that retaining the FeII state in such preferred compounds necessitates difficult and expensive syntheses in order to prevent the formation of stable compounds, such as crystalline FeIII phosphates, e.g., LiFeP2O7, or lithiated amorphous FeIII phosphate compounds, from which efficient delithiation is not significantly attainable.
In order to address these shortcomings of previously employed crystalline lithiated iron phosphates, e.g., olivine LiFePO4 and Nasicon compositions, the present invention entails the use of more widely available, stable non-lithiated FeIII compounds of not only crystalline conformation, but also of amorphous character, as primary active electrode components in the fabrication of lithium-ion electrochemical battery cells. A further departure from the previous art resides in the additional use, with significant improvement in economy as well as operational results, of the more readily occurring and economical hydrated forms of these stable iron phosphate compounds.
The electrode materials of the present invention may be employed in any of the rechargeable electrochemical battery cell fabrication styles commonly in use throughout the industry. For example, these iron phosphate compounds may be incorporated as active electrode component in the rigid metal casing compression style typified by the well-known xe2x80x9cbuttonxe2x80x9d battery, as well as in the semi-rigid or flexible film-encased laminated component polymer layer style of more recent development, such as is generally represented in FIG. 1 and more specifically described in U.S. Pat. No. 5,840,087. Complementary electrode materials, including lithium compounds and alloys which provide a source of mobile lithium ions during cell operation, may be selected from any such as are presently in use in current battery cell fabrication, as may separator and electrolyte components and compositions which have been widely described.
The active electrode materials of the invention typically comprise the positive electrode of a cell combination which reversibly incorporates lithium ions during electrical cell discharge, generally by ion insertion or intercalation into the iron phosphate structure. The useful iron phosphate compounds comprise substantially stable FeIII materials, including both crystalline and amorphous forms, as well as hydrates of these compounds. The preferred economical amorphous compounds may be readily obtained in hydrated form from commercial sources, e.g., Aldrich Chemical Company, Inc., Milwaukee, Wis., USA. Such pristine (as purchased) commercial compounds as FePO4.nH2O and Fe4(P2O7)3.nH2O, nominally comprising 1xe2x89xa6nxe2x89xa64, may be advantageously obtained as 100 nm particle materials which exhibit little agglomeration, the former having specific surface of about 18 to 30 m2/g.
Performance comparison of cells comprising analogous morphology variants of preferred active materials, e.g., pristine amorphous FePO4.nH2O, anhydrous FePO4 obtained from heating that material to about 400xc2x0 C., and crystalline FePO4 obtained from further heating and sintering above about 800xc2x0 C., reveals a graduated decrease in reversible cell capacity from the pristine form to the crystalline. The higher capacity of the hydrated material is believed to be attributable to the solvating effect of included water upon Li ion transport, while the least desirable performance of the crystalline material follows the great enlargement of particle size from the amorphous form at about 100 nm to the crystals at about 20 xcexcm.
Further improvement in performance of these hydrated amorphous active materials is achieved by intimate mixing with battery-grade conductive carbon, such as an Acetylene Black or Super-P, preferably ball-milling for periods in excess of about 15 min up to a limit of efficiency at about 120 min. Resulting increases in cell discharge capacity to about 90% of theoretical is believed to be attributable to improved electrical conductivity provided in the material by the intimate carbon coating of the particles, since no significant change in particle size was observed. Similar treatment of crystalline active material, on the other hand, resulted in a reduction of particle size to about 500 nm, as well as in an increase of conductivity, both attributing to improvement in discharge capacity.